70 THE CARBOHYDRATE ECONOMY OF CACTI. 



IX. CONSUMPTION OF CARBOHYDRATES DURING 



STARVATION. 



A very striking property of many of the cacti is their ability to survive 

 long periods of drought and starvation. A notable record of this has been 

 made by MacDougal et al. 1 on the desiccation of Echinocactus. This plant, 

 left in the diffuse light of a room without being given any water, after six 

 years had the same proportion of water as at the beginning, although it had 

 lost almost 30 per cent of its original weight. It has since been found that 

 similar conditions obtain in many other cacti. Here are given the records 

 of the water-loss and rate of carbohydrate depletion of two sets of Opuntia 

 phceacantha. For each series a large number of joints of the same age were 

 taken from one plant and placed in a dark room, the temperature of which 

 was maintained throughout the experiment at 28 C. The joints were 

 placed upon racks permitting free circulation of air. For over a year they 

 remained in apparently perfectly healthy condition. At the end of this 

 time a number of the joints were planted in soil and watered, within a week 

 they put out roots, and shortly thereafter developed vegetative buds. 



If a rather definite water-content is to be maintained during long 

 periods while a plant is actually losing large quantities of water there 

 must, of course, be a simultaneous loss of dry material through respira- 

 tion. As has been shown, this dry material is essentially of carbohydrate 

 nature. A considerable portion thereof is in the form of cellulose, however, 

 and from the investigations by MacDougal et al. it is evident that this 

 cellulose may be utilized as food material by the plant under circumstances 

 of stress. Added to the tendency of the plant to maintain its water-balance 

 by a concomitant loss of dry material is the factor that the burning of sugar 

 also results in the formation of water. For each gram of sugar, calculated 

 as C 6 Hi2O 6 , there is formed 0.6 gram of water. When the water-content 

 TABLE 34. f the plant is already high this factor is, of course, 



quite insignificant, but when the water-content is 

 low the water added thereto by the burning of sugar 

 may be of some significance in the economy of the 

 plant. Naturally, the maintenance of the water- 

 balance is entirely a relative matter, the rates at 

 which respiration and transpiration may proceed 

 and the plant still maintain its water-balance de- 

 pend upon the original condition. Thus, in order to maintain its original 

 water-balance and retaining a constant rate of respiration, the plant may 

 lose the following quantities of water at the water-content indicated in the 

 accompanying table. For each gram of dry material, calculated as sugar, 

 lost by respiration the water-loss equaled as shown in table 34. 



1 MACDOUGAL, D. T., E. R. LONG, and J. G. BROWN. End results of desiccation and 

 respiration in succulent plants. Physiological Researches, 1, 289-325, 1915. 



At water 

 content of 



p. ct. 

 60 

 75 

 80 

 90 



Water loss. 



grams. 

 2.1 

 3.5 

 4.5 

 9.6 



